COMPUTATIONAL MODELLING OF LIQUID JET IMPINGEMENT ONTO HEATED SURFACE

Quenching of heated surfaces through impinging liquid jets is of great im-portance for numerous applications like steel processing, nuclear power plants, automobile industries, etc. Therefore, computational modelling of the surface quenching through circular water jets impinging normally onto a heated flat surface has vital importance in order to reveal the physics of the quenching process. At first, a numerical model was developed for single jet impingement process. A conjugate heat transfer problem was solved implying consideration of both regions, one occupied by fluid (multi-phase flow consisting of water, vapor and ambient air) and one accommodating the solid surface within the same solution domain. Numerical simulations were performed in a range of relevant operating param-eters: jet velocities (2.5, 5, 7.5 and 10 m/s), water sub-cooling (75 K) and wall-superheat (650 K - 800 K) corresponding closely to those encountered in the industrial water jet cooling banks. Due to the high initial temperature of the surface, the boiling process exhibits strong spatial and temporal fluctuations. Its effect on the boundary layer profiles at the stagnation region at different time intervals are analyzed. The analysis reveals a highly distorted field of both mean flow and turbulence quantities. It represents an im-portant outcome, also with respect to appropriate model improvements. The different boiling characteristics are envisaged in detail to increase the level of understanding of the phenomena. The influence of the turbulent kinetic energy investigated at the boiling front as well as the jet-acceleration region has been studied. The physically relevant results are obtained and analyzed along with reference database provided experimentally by Karwa (2012, ‘Experimental Study of Water Jet Impingement Cooling of Hot Steel Plates,' Dissertation, FG TTD, TU Darmstadt). The intensive quenching process is consistent with the high rate of sub-cooling and high jet velocities. The surface temperature predicted by quenching model within the impingement region and the subsequent wall-jet region agrees reasonably well with the measurements, the outcome being particularly valid at higher jet velocities. However, a steep temperature gradient at the position corresponding to boiling threshold has not been captured under the condition of the numerical grid adopted. On the other hand, a reasonably good prediction of the wetting front propagation phenomena advocates the future development of the model. The high-intensity back motion of the vapor phase in the stagnation region at the earlier times of the water jet impingement can induce an appropriately high turbulence level, which could be accounted well by the turbulence model applied. The second part of the present work deals with multiple liquid jet impingement. When the multiple jets impact onto the heated surface, the heat flux is extracted from the surface by the mass flow rate. The heat flux is dependent on the several flow conditions and configurations of the nozzle array system. Therefore, one needs to study the nozzle array configuration along with several flow parameters for the better design of the cooling header system. Accordingly, the hydrodynamics of the multiple jets has been studied computationally realizing the need for optimum configuration of the nozzle array. The effects of the mass flow rate, target plate width and the turbulence produced due to the impingement were studied. Afterward, an analytical model is proposed for the quenching of the multiple jets system. It has been realized that, when jet impinges onto the surface at very high initial temperature, the film boiling may play a role in the heat transfer mechanism. Therefore, development has been made in the film-boiling model considering the effect of turbulence at the liquid jet stagnation region at the Leidenfrost point. The Leidenfrost point is the minimum temperature at which the film boiling can sustain. However, the vapor-liquid interface has the dynamic character; it oscillates with high frequency and causes the additional momentum diffusivity. Therefore, the need for introducing the effect of associated turbulence has been felt. The length and velocity scale of the turbulent structure has been approximated by assuming homogeneous turbulence. The new model for the heat flux and wall superheat yielded results agreeing well with published experimental results

Mathematical model of a constructal Coanda effect nozzle

This paper analyses the ACHEON Coanda effect nozzle for aircraft propulsion, based on the dynamic equilibrium of two jet streams. The ACHEON concept, and, in particular, the HOMER nozzle, which is its main component, are presented, together with the literature milestones from which the idea originally stems. A subsystem analysis inspired by the principles of Constructal Theory is presented for the current architecture. A mathematical model of a 2D case of the system is developed, focusing on the combined effect of the mixing of the two streams and the Coanda adhesion over a convex surface. A validation of the model is also reported, based on 2D CFD analyses, under the hypothesis of incompressible flow. Results highlight that, in spite of its relative simplicity, the model produces accurate results

A new aircraft architecture based on the ACHEON Coanda effect nozzle: flight model and energy evaluation

Purpose Aeronautic transport has an effective necessity of reducing fuel consumption and emissions to deliver efficiency and competitiveness driven by today commercial and legislative requirements. Actual aircraft configurations scenario allows envisaging the signs of a diffused technological maturity and they seem very near their limits. This scenario clearly shows the necessity of radical innovations with particular reference to propulsion systems and to aircraft architecture consequently. Methods This paper presents analyses and discusses a promising propulsive architecture based on an innovative nozzle, which allows realizing the selective adhesion of two impinging streams to two facing jets to two facing Coanda surfaces. This propulsion system is known with the acronym ACHEON (Aerial Coanda High Efficiency Orienting Nozzle). This paper investigates how the application of an all-electric ACHEONs propulsion system to a very traditional commuter aircraft can improve its relevant performances. This paper considers the constraints imposed by current state-of-the-art electric motors, drives, storage and conversion systems in terms of both power/energy density and performance and considers two different aircraft configurations: one using battery only and one adopting a more sophisticated hybrid cogeneration. The necessity of producing a very solid analysis has forced to limit the deflection of the jet in a very conservative range (±15°) with respect to the horizontal. This range can be surely produced also by not optimal configurations and allow minimizing the use of DBD. From the study of general flight dynamics equations of the aircraft in two-dimensional form it has been possible to determine with a high level of accuracy the advantages that ACHEON brings in terms of reduced stall speed and of reduced take-off and landing distances. Additionally, it includes an effective energy analysis focusing on the efficiency and environmental advantages of the electric ACHEON based propulsion by assuming the today industrial grade high capacity batteries with a power density of 207 Wh/kg. Results It has been clearly demonstrated that a short flight could be possible adopting battery energy storage, and longer duration could be possible by adopting a more sophisticated cogeneration system, which is based on cogeneration from a well-known turboprop, which is mostly used in helicopter propulsion. This electric generation system can be empowered by recovering the heat and using it to increase the temperature of the jet. It is possible to transfer this considerable amount of heat to the jet by convection and direct fluid mixing. In this way, it is possible to increase the energy of the jets of an amount that allows more than recover the pressure losses in the straitening section. In this case, it is then possible to demonstrate an adequate autonomy of flight and operative range of the aircraft. The proposed architecture, which is within the limits of the most conservative results obtained, demonstrates significant additional benefits for aircraft manoeuvrability. In conclusion, this paper has presented the implantation of ACHEON on well-known traditional aircraft, verifying the suitability and effectiveness of the proposed system both in terms of endurance with a cogeneration architecture and in terms of manoeuvrability. It has demonstrated the potential of the system in terms of both takeoff and landing space requirements. Conclusions This innovation opens interesting perspectives for the future implementation of this new vector and thrust propulsion system, especially in the area of greening the aeronautic sector. It has also demonstrated that ACHEON has the potential of renovating completely a classic old aircraft configuration such as the one of Cessna 402

COMPUTATIONAL MODELLING OF LIQUID JET IMPINGEMENT ONTO HEATED SURFACE

Quenching of heated surfaces through impinging liquid jets is of great im-portance for numerous applications like steel processing, nuclear power plants, automobile industries, etc. Therefore, computational modelling of the surface quenching through circular water jets impinging normally onto a heated flat surface has vital importance in order to reveal the physics of the quenching process. At first, a numerical model was developed for single jet impingement process. A conjugate heat transfer problem was solved implying consideration of both regions, one occupied by fluid (multi-phase flow consisting of water, vapor and ambient air) and one accommodating the solid surface within the same solution domain. Numerical simulations were performed in a range of relevant operating param-eters: jet velocities (2.5, 5, 7.5 and 10 m/s), water sub-cooling (75 K) and wall-superheat (650 K - 800 K) corresponding closely to those encountered in the industrial water jet cooling banks. Due to the high initial temperature of the surface, the boiling process exhibits strong spatial and temporal fluctuations. Its effect on the boundary layer profiles at the stagnation region at different time intervals are analyzed. The analysis reveals a highly distorted field of both mean flow and turbulence quantities. It represents an im-portant outcome, also with respect to appropriate model improvements. The different boiling characteristics are envisaged in detail to increase the level of understanding of the phenomena. The influence of the turbulent kinetic energy investigated at the boiling front as well as the jet-acceleration region has been studied. The physically relevant results are obtained and analyzed along with reference database provided experimentally by Karwa (2012, ‘Experimental Study of Water Jet Impingement Cooling of Hot Steel Plates,' Dissertation, FG TTD, TU Darmstadt). The intensive quenching process is consistent with the high rate of sub-cooling and high jet velocities. The surface temperature predicted by quenching model within the impingement region and the subsequent wall-jet region agrees reasonably well with the measurements, the outcome being particularly valid at higher jet velocities. However, a steep temperature gradient at the position corresponding to boiling threshold has not been captured under the condition of the numerical grid adopted. On the other hand, a reasonably good prediction of the wetting front propagation phenomena advocates the future development of the model. The high-intensity back motion of the vapor phase in the stagnation region at the earlier times of the water jet impingement can induce an appropriately high turbulence level, which could be accounted well by the turbulence model applied. The second part of the present work deals with multiple liquid jet impingement. When the multiple jets impact onto the heated surface, the heat flux is extracted from the surface by the mass flow rate. The heat flux is dependent on the several flow conditions and configurations of the nozzle array system. Therefore, one needs to study the nozzle array configuration along with several flow parameters for the better design of the cooling header system. Accordingly, the hydrodynamics of the multiple jets has been studied computationally realizing the need for optimum configuration of the nozzle array. The effects of the mass flow rate, target plate width and the turbulence produced due to the impingement were studied. Afterward, an analytical model is proposed for the quenching of the multiple jets system. It has been realized that, when jet impinges onto the surface at very high initial temperature, the film boiling may play a role in the heat transfer mechanism. Therefore, development has been made in the film-boiling model considering the effect of turbulence at the liquid jet stagnation region at the Leidenfrost point. The Leidenfrost point is the minimum temperature at which the film boiling can sustain. However, the vapor-liquid interface has the dynamic character; it oscillates with high frequency and causes the additional momentum diffusivity. Therefore, the need for introducing the effect of associated turbulence has been felt. The length and velocity scale of the turbulent structure has been approximated by assuming homogeneous turbulence. The new model for the heat flux and wall superheat yielded results agreeing well with published experimental results

Mathematical modelling of a two streams coanda effect nozzle

This paper analyses the ACHEON Coanda effect based propulsion nozzle for aircraft propulsion based on the dynamic equilibrium of two jet streams. It presents a large bibliographic analysis and the ACHEON concept and, in particular, the HOMER Nozzle, that is its main component. The Constructal optimization process that allows defining this architecture has presented. A preliminary mathematical model of a 2D case of the system has presented, focusing on the combined effect of the mixing effect of the two streams and the Coanda Effect Adhesion over a convex surface. A CFD preliminary validation has presented in uncompressible regime. The results have been evaluated in 2D cases

Wake Formation Flow Physics and Boundary Layer Analysis on the Sides of the Isosceles Triangular Cylinder with Apex Pointing Downstream

Boundary layer interaction with downstream flow structures was numerically studied to find the region of inactivity behind an 75° isosceles triangular cylinder with apex pointing downstream at intermediate Reynolds numbers (Re = 520, 640, 840 and 1040). The Standard k-ε model in OpenFOAM was used in the study. Numerical results were validated against Particle Image Velocimetry data. Results revealed the stable region of inactivity characterized by low turbulent kinetic energy and vorticity. The onset of secondary vortex and separation point, independent of Reynolds number, was identified. The onset of the secondary vortex was located at (x = 2 mm) from the base and (y = 1.5 mm) from the apex on either side of the cylinder. The ratio of modulus of absolute primary z-component of vorticity, |ω_z^1 |, to the modulus of secondary z-component of vorticity, |ω_z^2 |, was found to be approximately equal to 1.2. This ratio is invariant of the Reynolds number of the study. These findings have practical implications. The unique properties of the inactivity region forms an ideal location that can be used for injecting fluid, placing measurement probe, active flow control and drag reduction. The research problem is formulated in the introduction. Literature is reviewed next providing the background. Details about the range of parameters, governing equations, numerical study details and software used are given in the methodology section. The results section gives the numerical results, verified by mesh refinement test and validated against experimental results. The results are finally discussed in the next section

Preliminary implementation study of ACHEON thrust and vector electrical propulsion on a STOL light utility aircraft

One of the best airplanes ever realized by the European Aircraft industry was the Dornier Do 28D Skyservant, an extraordinary STOL light utility aircraft with the capability to carry up to 13 passengers. It has been a simple and rugged aircraft capable also of operating under arduous conditions and very easy and simple maintenance. The architecture of this airplane, which has operated actively for more than 20 years, is very interesting analyzing the implementation of a new propulsion system because of the unusual incorporation of two engines, as well as the two main landing gear shock struts of the faired main landing gear attached to short pylons on either side of the forward fuselage. This unconventional design allows an easy implementation of different propulsion units, such as the history of different experimental versions allowed. This paper presents the preliminary definition of an increased performance cogeneration system for optimizing the energy efficiency and maximizing the thrust of ducted fan propeller. It then produces an effective design of the ACHEON nozzle for such an aircraft, the definition of the optimal positioning for stability and efficiency. In conclusion, it analyses the expected performances of the resulting aircraft architecture. Outstanding results allows verifying an effective possibility of implementing the ACHEON Coanda effect thrust and vector propulsion system on real aircraft

Multifunctional unmanned reconnaissance aircraft for low-speed and STOL operations

This paper presents a novel UAS (Unmanned Aerial System) designed for excellent low speed operations and VTOL performance. This aerial vehicle concept has been designed for maximizing the advantages by of the ACHEON (Aerial Coanda High Efficiency Orienting-jet Nozzle) propulsion system, which has been studied in a European commission under 7th framework programme. This UAS concept has been named MURALS (acronym of Multifunctional Unmanned Reconnaissance Aircraft for Low-speed and STOL operation). It has been studied as a joint activity of the members of the project as an evolution of a former concept, which has been developed during 80s and 90s by Aeritalia and Capuani. It has been adapted to host an ACHEON based propulsion system. In a first embodiment, the aircraft according to the invention has a not conventional shape with a single fuselage and its primary objective is to minimize the variation of the pitching moment allowing low speed operations. The shape with convex wings has been specifically defined to allow a future possibility of enabling stealth operations. Main objective of the design activity has been focused on low speed flight, very short take off and landing, and a control possibility by mean of two mobile surfaces in the front canard, which allow changing the pitch angle, and allows an almost complete plane control in combination with an ACHEON variable angle of thrust propulsion system. The design considers has been specifically to allow flying at a speed which is lower than 12 m/s with an high angle of attach (over 7°), without losses in terms of manoeuvrability and agility. These features allow innovative uses such as road monitoring, and police support and are characterized by a breakthrough performance level. A complete optimal sizing of the aircraft has been performed, together with an effective performance analysis, which allows identifying the strong points and the potential problems of the project. An effective energy analysis has been performed also. An effective prototyping is expected in about one year

Preliminary implementation study of ACHEON thrust and vector electrical propulsion on a STOL light utility aircraft

One of the best airplanes ever realized by the European Aircraft industry was the Dornier Do 28D Skyservant, an extraordinary STOL light utility aircraft with the capability to carry up to 13 passengers. It has been a simple and rugged aircraft capable also of operating under arduous conditions and very easy and simple maintenance.The architecture of this airplane, which has operated actively for more than 20 years, is very interesting analyzing the implementation of a new propulsion system because of the unusual incorporation of two engines, as well as the two main landing gear shock struts of the faired main landing gear attached to short pylons on either side of the forward fuselage. This unconventional design allows an easy implementation of different propulsion units, such as the history of different experimental versions allowed.This paper presents the preliminary definition of an increased performance cogeneration system for optimizing the energy efficiency and maximizing the thrust of ducted fan propeller. It then produces an effective design of the ACHEON nozzle for such an aircraft, the definition of the optimal positioning for stability and efficiency. In conclusion, it analyses the expected performances of the resulting aircraft architecture.Outstanding results allows verifying an effective possibility of implementing the ACHEON Coanda effect thrust and vector propulsion system on real aircraft.</p